Liquid crystal devices using liquid crystalline compounds (in the present specification, the term of the liquid crystalline compound is used as a generic name of the compound showing liquid crystal phases and the compound not showing liquid crystal phases but useful as ingredients of the liquid crystal compositions) are broadly used in displays of clocks, watches and electronic calculators, word processors and the like. To these display devices, an optical anisotropy, a dielectric anisotropy and the like of the liquid crystalline compound are applied.
As the liquid crystal phases, a nematic liquid crystal phase, a smectic liquid crystal phase, and a cholesteric liquid crystal phase are used, and particularly, the nematic liquid crystal phase is broadly utilized. As display modes, there are a dynamic scatter (DS) type, a deformation type of an aligning phase (DAP), a guest/host (GH) type, a twisted nematic (TN) type, a super twisted nematic (STN) type, a thin film transistor (TFT) type and the like.
Although the liquid crystalline compounds used in these display modes must show liquid crystal phases at broad temperature ranges, mainly at room temperature, they must be sufficiently stable under conditions that the display devices are used, and they must have characteristics enough to drive the display devices, a single liquid crystalline compound satisfying these conditions is still not found. For these reasons, a few kinds or several tens of kinds of liquid crystalline compounds are conventionally mixed to prepare the liquid crystal compositions satisfying desired properties. It is required that these liquid crystal compositions are stable to moisture, light, heat and air which exist in normal conditions using the display devices, stable to electric field and electromagnetic radiation, and chemically stable to mixed compounds. Further, it is required that these liquid crystal compositions show proper valuer physical properties such as a refractive index anisotropy value (.DELTA. n) and a dielectric anisotropy value (.DELTA. .epsilon.) of the liquid crystal compositions dictate the display modes or the forms of the display devices. Further, it is important that each ingredient of these liquid crystal compositions has mutually good solubility.
In recent years, various environment using the liquid crystal devices requires the liquid crystal compositions showing liquid crystal phases at a higher temperature. To achieve the requirement, the ingredient may be constituted from the compounds having high transition temperature of an isotropic phase. Such compounds are known from the compounds represented by formula (a) of Japanese Patent Publication No. 62-46527 and formula (b) of Japanese patent Publication No. 4-28693. ##STR1##
These compounds have a high transition temperature of an isotropic phase and relatively low viscosity, but the compounds have problems that the compatibility with the other ingredients in the composition is not enough, so that the compounds should be used in limited amounts.
Objects of the present invention are to solve the problems of the above conventional techniques, and to provide liquid crystalline compounds having excellent compatibility with the other liquid crystal materials and stability, low viscosity, and a high transition temperature of an isotropic phase; liquid crystal compositions comprising the compounds and liquid crystal devices constituted by using the liquid crystal compositions.